1. Field of the Invention
The invention relates to a method of determining the nuclear magnetization distribution in a body from magnetic resonance signals which are excited in sequences in such body located in a stationary magnetic field, a sequence comprising at least one high-frequency electromagnetic pulse produced by means of a reference signal, which pulse excites a magnetic resonance signal in the body which is detected in the presence of the reference signal.
The invention further relates to an arrangement for determining a nuclear magnetization distribution in a part of a body, the arrangement comprising means for producing a stationary magnetic field, means for producing magnetic field gradients, means for producing a reference signal, transmitter apparatus for transmitting a high-frequency electromagnetic pulse formed in the transmitter apparatus by means of the reference signal, a receiver apparatus for receiving and detecting by means of the reference signal a resonance signal which is excited in the body by such pulse, and a processing apparatus for processing the detected resonance signals.
The term "nuclear magnetization distribution" is to be interpreted in a broad sense, so that inter alia, terms like "spin nuclei density distribution", "longitudinal relaxation time (T.sub.1) distribution", "transversal relaxation time (T.sub.2) distribution" and "nuclear magnetic resonance frequency spectrum distribution" (NMR position-dependent spectroscopy) are all to be understood to be included in the term "nuclear magnetization distribution".
2. Description of the Related Art
Such a method and such an arrangement are known from the article by Locher in "Philips Technical Review", Volume 41, 1983/84, No. 3. In such a method, a body to be examined is arranged in a stationary magnetic field B.sub.O, whose direction coincides with the axis of a stationary Cartesian coordinate system. The magnetic field B.sub.O has the result that a small excess quantity of the spin nuclei present in the body has the same direction as the field B.sub.O. The fact that there is only a small excess quantity with respect to the theoretically possible saturation value (all spins having the same direction) is due to thermal movement, as a result of which a large number of spin nuclei assume a direction opposite to the field. The small excess quantity is to be interpreted, considered macroscopically, as a magnetization M of the body or as a small polarization of the spin nuclei present in the body. After irradiation of the body arranged in the magnetic field by a high-frequency electromagnetic pulse, which should have a given frequency, the magnetization M is unbalanced and will perform a precession movement around the magnetic field B.sub.O. When the precession movement is considered from a rotating Cartesian coordinate system (x, y, z) whose z axis coincides with the z axis of the said stationary Cartesian coordinate system, and when the angular velocity of the rotating Cartesian coordinate system is chosen to be equal to the angular frequency of the high-frequency electromagnetic pulse, the magnetization M is to be interpreted as a vector, which due to the irradiation moves in a plane at right angles to the direction of irradiation. The component of the magnetization M at right angles to the z axis, i.e. the so-called transversal magnetization, has the result that after irradiation a resonance signal is obtained. For the angular frequency of the high-frequency electromagnetic pulse, the relation .omega..sub.O =.gamma..B.sub.O must hold in order that the spin nuclei will perform a "Larmor" precession movement around the magnetic field M, where .omega..sub.O is the precession angular frequency (of protons if a spin nuclei density distribution of protons is desired), .gamma. is the gyromagnetic ratio (of the proton) and B.sub.O is the strength of the stationary magnetic field. The high-frequency electromagnetic pulses are formed in a transmitter apparatus in which the pulse is produced by modulation of a reference signal which is supplied b an oscillator and has a frequency (in the proximity of) .omega..sub.O with a low-frequency signal determining the pulse shape, after which the pulse is transmitted by the transmitter apparatus by means of a transmitter coil located near the body. The magnitude of the transversal magnetization is determined by the surface area below the pulse. A 90.degree. pulse is concerned when the magnetization rotates through 90.degree. with respect to the magnetic field B.sub.O through the pulse. The resonance signal produced by the high-frequency electromagnetic pulse can be detected with a receiver coil by means of a receiver apparatus with the aid of so-called double phase-sensitive detection, the reference signal and a signal obtained by shifting the phase of the reference signal through 90.degree. being employed. It can be simply demonstrated that the detected resonance signal, except for a constant factor, represents the transversal magnetization. When resonance signals are produced in a given manner in sequences in the body, it is possible, for example, to reconstruct with the information obtained from the resonance signals a proton density distribution by processing means. For example, in general and also from the said article by Locher the so-called spin echo sequence is known for producing the resonance signals and the so-called "Fourier zeugmatography" is known for image formation from the resonance signals. For example, in order to obtain information about a local proton density in the body, it is necessary not only to irradiate the body by high-frequency electromagnetic pulses, but also to apply magnetic field gradients, whose field direction coincides with the magnetic field B.sub.O. It is shown in the article by Locher that there is a direct relation between a frequency in the resonance signal and a local proton density (position). By the application of a magnetic field gradient at the magnetic field B.sub.O having a given gradient direction, a resonance signal is obtained whose spectrum (Fourier transformation of the resonance signal) is, for example, the image of the proton density in the gradient direction. Bidimensional Fourier zeugmatography utilizes this phenomenon. Two gradients are applied for encoding the position-dependent proton density in the body, a so-called preparation gradient, whose gradient direction coincides with the y axis of, for example, a stationary Cartesian coordinate system (x, y, z), assuming in each sequence a different value and the so-called measurement gradient, whose gradient direction coincides with the x axis, having the same course for each sequence. In cooperation with high-frequency electromagnetic pulses (mostly selectively, i.e. with another magnetic field gradient, whose direction coincides with the z axis, only spin nuclei are selected in a layer at right angles to the z axis), resonance signals are obtained, which after sampling thereof yield sample values. After bidimensional (discrete) Fourier transformation (for example "Fast Fourier transformation") in the processing apparatus, image elements are obtained from the sample values formed and these image elements together constitute an image, in this case a proton density of a slice of the body. By a suitable choice of magnetic field gradients and high-frequency electromagnetic pulses, tridimensional proton density distributions of frequency spectra per image element (from which an interpretation of the metabolic state of the image element can be given) can be obtained.
Due to different causes, in an image obtained by, for example, Fourier zeugmatography, unwanted components occur in the resonance signals which become manifest in the image as interference lines and/or ghost images. The usual NMR equipment has the disadvantage that stringent requirements must be imposed thereon; for example, 180.degree. pulses should have a high degree of perfection and phase shifts between sequences should be optimum in order to prevent that a resonance signal formed in a sequence does not "leak through" to a following sequence in which the next resonance signal is produced. Coherent interferences are concerned here. Other interference sources also give rise to coherent interferences. Interference sources are inter alia: cross-talk in the high-frequency electronic circuits of transmitter and receiver, coherence between the reference signal and the clock of analog-to-digital converters in the processing means, periodical variations in the gradient strengths, mechanical oscillations of the NMR system.
It should noted that it is known from U.S. Pat. No. 3,968,424 to influence spectra by means of a magnetic resonance arrangement employing high-frequency electromagnetic pulses. The arrangement described in such patent is designed to determine NMR spectra of a sample to be analyzed. For this purpose, pulse trains of high-frequency electromagnetic pulses are produced, while unequal phases can be given to successive pulses. FID signals originating from many successive pulses are averaged in time and a spectrum of the sample is determined from the resulting signal by means of a Fourier transformation. However, no mention is made in such patent of image formation by Fourier transformation of resonance signals which are produced in a number of successive sequences, or of coherent interferences occuring between the sequences which give rise to image artefacts, such as interference lines, phantom images or conspicuous dots in an image. In such patent the object is to avoid artefacts in spectra which are caused by too rapid a succession of pulses. Smearing-out of image artefacts over an image is not achieved.